Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181344

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
744 AM CST Sat Nov 18 2017

732 AM CST

Still monitoring convective trends here in the near term, mainly
south of I-80 but more closely along/south of a Pontiac IL to
Wheatfield IN line. In this location, strong LLJ and isentropic
ascent is resulting in more robust thunderstorm development. This
strong forcing is helping to offset only weak instability in
place, with several stronger storms currently moving northeast
through central IL. Radar signatures and environment would
suggest large hail could be a threat, and continue to monitor
mainly this hazard as any surface based instability remains well
south of the area. However, latest radar signatures continue to
show a weakening trend with these northward moving storms and this
makes sense given the southern CWA is right on the line of this
instability axis. Focus over the next 1-2 hours will be in this
location for mainly small hail and an isolated storm possibly
capable of producing hail up to 1 inch. Additionally, moderate to
briefly heavier rain will likely occur with these stronger storms.

After this, focus will shift back to the north, along/north of
I-88 where a transition over to a wintry mix will definitely be
possible. This is already noted across southwest/south WI, where a
mix of rain, snow, and sleet are occurring.



345 AM CST

Deepening low pressure system moving through the region today,
presenting plenty of near term forecast concerns. Precip trends,
including morning thunderstorms and midday/afternoon rain/wet snow
mix, falling temps and very windy conditions all the main foci
for the next 12-18 hours. Lake Shore Flood Advisory will be issued
for Indiana shore of Lake Michigan for this afternoon and
tonight, due to strong north winds producing large waves leading
to potential flooding and beach erosion.

Surface low pressure along the KS/MO border early this morning,
will deepen in response to strong mid-level height falls
associated with a strengthening upper trough which will propagate
across the region today. The low is progged to lift quickly
northeast across central IL and into northern IN by midday, and to
Lake Erie by early this evening. Strong forcing, including
isentropic ascent of warm/moist air atop strengthening surface
stationary/warm frontal trough in advance of the low and enhanced
by upper divergence in left exit region of 140+ knot upper jet
streak, is expected to result in rain expanding across the area
again early this morning. Current radar and lightning detection
depicts thunderstorms occurring across the southern tier or so of
cwa counties where RAP mesoanalysis indicated 500+ J/kg of MUCAPE
along and south of the stationary frontal zone. Thunderstorm
potential is expected to persist across these southern counties
this morning, before instability is shunted off to the east of the
area with the passage of the surface low late this morning.
Locations south of a Pontiac-Kankakee-Rensselaer line may see
1-1.50 inch rain amounts from this convection.

Various model guidance depicts deformation zone precipitation
lingering across the forecast area into this afternoon in
association with the elevated baroclinic zone north of the surface
front and low track, with cooling of the thermal column expected
as northerly winds increase and pull colder air in behind the
deepening low. Morning temperatures in the low 40`s to mid 50`s
north to south will fall into the mid-upper 30`s across most of
the area by afternoon, with forecast soundings supporting a
mix/change to some wet snow especially across northern IL portions
of the cwa during the late morning and afternoon. Ground/air
temps are expected to remain mild enough, with surface wet-bulb
temps in the mid 30`s such that little if any accumulation is
expected, perhaps a slushy tenth or two on some grassy surfaces
where precip is briefly intense enough. Precip will taper off and
end from west to east this afternoon into early this evening.

The other notable weather concern will be the development of very
windy conditions by late morning/midday, as northerly winds
increase to 20-30 mph with gusts to 40-45 mph in the wake of the
deepening low. Not only will these winds make for uncomfortable
wind chills in the 20`s, but they will also result in large waves
building on southern Lake Michigan, which will likely cause lake
shore flooding issues along the south (Indiana) shore this
afternoon into tonight. Based on this, a Lakeshore Flood Advisory
for Lake and Porter counties from noon today through 3 am CST
Sunday morning.



346 AM CST

Sunday through Friday...

Dry weather is expected through much of the coming week, with
variable temperatures as a series of upper troughs and surface low
pressure systems move through the northern Lakes.

High pressure builds from the Plains through the Ohio Valley and
Gulf Coast states Sunday into Monday, in the wake of Saturday`s
deep low pressure system. This results initially in a chilly day
Sunday, with winds still somewhat blustery early as the center of
the high passes south of the area. Winds turn southwesterly Monday
however, as an area of low pressure moves across southern Canada
and passes north of the Lakes Tuesday. This brings temps back into
the 40`s across the cwa Monday-Tuesday, before a cold front trails
across the region later in the day Tuesday. Colder but dry
conditions then develop mid-week as high pressure builds back into
the area from the west.

Moderation is then expected again by the end of the week, with
precipitation chances slowly increasing into the weekend, as
models develop an upper trough across the Great Lakes and Midwest
region. Nighttime temps in the mid-30`s and daytime temps in the
40`s would suggest any precip by Friday and Saturday would be mainly
in the form of rain.



For the 12Z TAFs...

610 am...Several forecast concerns this period including winds...
cigs and precip timing.

Cold front has moved south of the terminals and winds have shifted
to the north/northeast. Forecast soundings suggest winds will
remain north/northeast through at least mid morning and then will
slowly turn more northerly by late morning and eventually to the
north/northwest this afternoon. Speeds/gusts will steadily
increase this morning with speeds into the 15-20kt range with
gusts 25-30kts. Winds will slowly begin to diminish this evening
but will likely remain gusty through much of tonight.

Showers continue to expand in coverage across the area and these
will continue this morning with a period of prevailing rain likely
from late morning through early afternoon. As colder air spreads
into the region today...there is some potential for the rain to
mix with snow as it ends. Confidence is fairly low for how much
snow may mix in and only carried mention of rain/snow mix at rfd.
Scattered thunderstorms are expected across central IL and these
may continue over the same areas for much of this morning. Its
possible an isolated thunderstorm may develop as far north as the
Interstate 80 corridor and possibly affect gyy. Confidence too low
for mention with this forecast and trends will need to be

Visibilities are slowly improving as the cold front moves south
and cigs have also slightly improved but expect prevailing ifr
cigs for much of this morning...gradually lifting to mvfr this
afternoon. Lower cigs can be expected ahead of the front as it
moves south across east central IL and northwest Indiana. Mvfr
cigs may persist for much of this evening...slowly scattering out
from west to east tonight. cms



150 am...A cold front will move south across Lake Michigan early
this morning shifting winds northerly. The gradient will quickly
tighten as low pressure moves from Missouri this morning to near
Lake Erie this evening. These northerly winds will increase to
gales by late morning with 45kts likely on the south end of the
lake this afternoon. The gales will slowly diminish from north to
south late tonight into early Sunday morning. A large area of high
pressure will move across the southern U.S. Sunday into Monday.
As it passes south of the lake...northwesterly winds Sunday
morning will shift southwesterly Sunday night. Low pressure will
move across Ontario Monday night and the gradient will tighten
ahead of this low with a period of southwesterly gales expected
late Monday afternoon through Tuesday morning. A trailing cold
front will move across the lake Tuesday afternoon and then large
high pressure will move across the region next Wednesday. cms


IN...Lakeshore Flood Advisory...INZ001-INZ002...noon Saturday to 3 AM

LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 until 6 AM Sunday.




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